1. Field of Invention
The invention relates to an electrophotographic image forming apparatus, such as a laser printer.
2. Description of Related Art
Electrophotographic image forming apparatus are well known in the art. These devices, such as a laser printer, typically include a photosensitive drum, a charger, a laser scanner, a developing roller, and a transfer roller. After the surface of the photosensitive drum is uniformly charged by the charger, the surface of the photosensitive drum is irradiated with a laser beam emitted from the laser scanner, and an electrostatic latent image is formed based on predetermined image data.
Toner carried on the developing roller is supplied to the electrostatic latent image formed on the surface of the photosensitive drum. The toner deposited on the surface of the photosensitive drum is transferred to a sheet passing between the photosensitive drum and the transfer roller.
Paper dust is deposited on the surface of a photosensitive drum when a sheet passes between the photosensitive drum and a transfer roller. If any paper dust remains on the photosensitive drum, a charger is prevented from uniformly charging the surface of the photosensitive drum and print quality deteriorates. An image forming device structured according to the apparatus of this invention efficiently removes paper dust deposited on the surface of the photosensitive drum. This is preferably achieved using an electrically conductive brush.
Generally, when paper dust on a photosensitive drum is electrically collected by applying a bias voltage to a conductive brush and if a potential difference between the bias voltage applied to the conductive brush and the surface potential of the photosensitive drum is too great, electric discharge may occur between the conductive brush and the photosensitive drum. Therefore, the bias voltage applied to the conductive brush should be set so as not to differ greatly from the surface potential of the photosensitive drum.
The surface potential of the photosensitive drum varies greatly depending on changes of a transfer current of a transfer roller and the on/off state of a transfer bias. When the bias voltage applied to the conductive brush does not differ greatly from the surface potential of the photosensitive drum, the high-low relationship between the voltage applied to the conductive brush and the surface potential of the photosensitive drum may be reversed. In such a case, paper dust collected by the conductive brush will be released to the surface of the photosensitive drum.
To solve this problem, the surface potential of the photosensitive drum should be kept stabilized at any given time and the relationship between the surface potential of the photosensitive drum and the bias voltage applied to the conductive brush should be kept constant. To that end, a discharge lamp can be provided downstream from the transfer roller and upstream from the conductive brush with respect to the rotation direction of the photosensitive drum.
Providing a discharge lamp is advantageous in that the potential difference between the surface potential of the photosensitive drum and the bias voltage applied to the conductive brush is stabilized and that the conductive brush can stably collect paper dust. However, such a discharge lamp has recently been eliminated for design simplicity and cost reduction.
According to this invention, a diode is provided between the conductive brush and a power source for the conductive brush. Thus, paper dust deposited on the photosensitive drum can be electrically collected in a stable manner without the need for providing a discharge lamp. The diode is provided to prevent current flow from the photosensitive drum to the power source.
Even when the surface potential of the photosensitive drum varies depending on changes of the transfer current and the on/off state of the transfer bias, and even when the high-low relationship between the bias voltage applied to the conductive brush and the surface potential of the photosensitive drum is reversed, the diode prevents current flow from the photosensitive drum to the conductive brush. Therefore, no potential difference is caused between the photosensitive drum and the conductive brush, and the paper dust remains held by the conductive brush.
Because the paper dust collected by the conductive brush is not released to the photosensitive drum, the potential difference between the bias voltage applied by the power source to the conductive brush and the surface potential of the photosensitive drum can be minimized.
In one embodiment of the apparatus of the present invention, the bias voltage applied to the conductive brush is set at 400 V so as to differ by 100 V from the surface potential of the photosensitive drum set at 300V. With such a potential difference, no electric discharge is generated between the conductive brush and the photosensitive drum, and the ability of the conductive brush to remove paper dust may be improved.
The bias voltage applied to the conductive brush is set between the initial potential of the photosensitive drum charged by the charger and the potential of the unexposed portion of the photosensitive drum after the transfer of a visualized image to the sheet. However, when the transfer bias is off, the surface potential of the photosensitive drum may possibly become approximately 900 V and differ greatly from the bias voltage (400 V) applied to the conductive brush. In such a case, the diode provided between the conductive brush and the power source prevents current flow from the conductive brush to the power source. Thus, no potential difference is caused between the conductive brush and the photosensitive drum.
When the transfer bias is off, there is no sheet between the photosensitive drum and the transfer roller and no paper dust adheres to the photosensitive drum. In this case, if a potential difference is caused between the conductive brush and the photosensitive drum, the paper dust collected by the conductive brush could be released to the photosensitive drum. In this embodiment, however, no potential difference is caused between the conductive brush and the photosensitive drum, and thus the paper dust collected by the conductive brush remains held in place. Therefore, paper dust removing ability may be maintained without any control by a discharge lamp of the surface potential of the photosensitive drum. The apparatus of this embodiment of the present invention is advantageous in reducing the manufacturing cost of an image forming apparatus in that no discharge lamp is required for the structure.
By making the brush itself electrically conductive, for example, by dispersing conductive particles, such as carbon particles, or conductive fillers into the brush, paper dust deposited on the photosensitive drum can be collected physically as well as electrically. Thus, the ability of the brush to remove paper dust can be improved.
By setting the volume resistance of the conductive brush at less than 106 xcexa9-cm, a potential difference great enough to allow the brush to electrically collect paper dust is obtained. It is preferable that the conductive brush is made of an acrylic resin into which conductive particles or fillers are dispersed. Although the brush can be made conductive by coating its surface with metal, the metal-coated brush may become too firm and rub strongly against the surface of the photosensitive drum. Strong abrasion will aggravate filming on the photosensitive drum with paper dust or toner. However, if the brush is too soft, its ability to remove paper dust will be reduced.
For these reasons, it is preferable to use a brush made of an acrylic resin into which conductive particles or fillers are dispersed. With this structure, the brush is made moderately firm and can offer sufficient paper dust removing ability while suppressing filming.
The brush is structured such that its length may be 6 mm or more and its engaging amount against the photosensitive drum may be 1 mm or more. When the length of the brush is less than 6 mm and the engaging amount of the brush is less than 1 mm, the brush may rub, at its tip, against the surface of the photosensitive drum and may be likely to cause filming on the photosensitive drum.
On the other hand, as shown in one embodiment, when the length of the brush is 6 mm or more and the engaging amount of the brush is 1 mm or more, the brush makes contact with the surface of the photosensitive drum with its tip curved slightly. Accordingly, the brush may offer a sufficient paper dust removing ability while suppressing filming on the photosensitive drum.
The fiber density of the brush may preferably be more than 7.75 kf/cm2. When the fiber density of the brush is 7.75 kf/cm2 or less, paper dust is likely to pass through the brush. When the fiber density of the brush is more than 7.75 kf/cm2, the brush can satisfactorily collect paper dust. Accordingly, the ability of the brush to collect paper dust may be improved.
The fiber thickness of the brush may preferably be approximately 330 dt/48 f or less. When the fiber thickness of the brush is more than 330 dt/8 f, the brush may become too firm and may likely cause filming on the photosensitive drum. A brush satisfying the above-described requirements may offer an extremely high ability to remove paper dust deposited on the surface of the photosensitive drum.